Lysozyme is an antimicrobial protein that functions as a defense mechanism within the innate immune system. Alexander Fleming discovered it in 1922, noting its capacity to dissolve bacteria and naming it for its lytic action. This enzyme is found widely across the biological world, from plants and viruses to animals, where it protects against invading pathogens. Lysozyme’s primary role involves hydrolyzing the structural components of bacterial cell walls, safeguarding against infection.
Molecular Composition
Lysozyme is a small, stable protein, typically consisting of a single polypeptide chain of approximately 129 to 130 amino acid residues. The most studied form, hen egg-white lysozyme, has a molecular weight of around 14.3 kilodaltons (kDa). Its compact, globular structure is maintained by four disulfide bonds, which create cross-links that enforce a rigid tertiary fold.
The protein adopts a fold composed of two distinct structural domains, which form a prominent cleft on its surface. This cleft is the active site, and its specific shape is necessary for binding to the target molecule on the bacterial surface. Lysozyme is classified as a glycoside hydrolase, an enzyme that catalyzes the breakdown of glycosidic bonds using water. The enzyme’s high stability allows it to remain active across a wide range of pH and temperatures.
Sources in Nature
Lysozyme is distributed broadly throughout nature, but it is concentrated in bodily fluids and natural products requiring antimicrobial protection. One of the richest and most commercially utilized sources is hen egg white, where the enzyme protects the developing embryo from microbial contamination. This abundance is why hen egg white lysozyme is often used as a model system for studying protein structure and function.
Within the human body, lysozyme is an integral component of external secretions that form the first line of defense against environmental pathogens. High concentrations are found in tears, saliva, and mucus, providing antimicrobial protection to the eyes, oral cavity, and respiratory tract. Human milk contains lysozyme at concentrations thousands of times greater than those found in cow’s milk, demonstrating its importance in conferring innate immunity to infants.
The Bactericidal Mechanism
The primary target of lysozyme is peptidoglycan, a complex polymer that forms the rigid structure of bacterial cell walls. This structure is particularly thick and exposed in Gram-positive bacteria, making them highly susceptible to the enzyme. Peptidoglycan is constructed from long chains of alternating sugar derivatives: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
Lysozyme acts as a specific molecular scissor, binding to the peptidoglycan chain within its active site cleft. The enzyme catalyzes the hydrolysis of the \(\beta\)-(1→4)-glycosidic bond, which links NAM to the adjacent NAG residue. This enzymatic process involves two specific amino acid residues, Glutamic acid 35 and Aspartic acid 52, which facilitate the cleavage of this chemical bond.
The continuous breaking of these bonds rapidly degrades the structural integrity of the bacterial cell wall. Since the cell wall resists the high internal osmotic pressure of the bacterium, its weakening causes catastrophic failure. The cell absorbs water until it ruptures, a process known as osmotic lysis, which kills the bacterium. While Gram-negative bacteria possess a protective outer membrane, lysozyme can still exhibit antibacterial effects if the outer membrane is compromised.
Industrial and Clinical Applications
The antimicrobial property of lysozyme has been harnessed for commercial and therapeutic purposes. In the food industry, purified lysozyme is widely used as a natural preservative to extend shelf life and ensure product safety. It is incorporated into products like semihard cheeses to prevent the growth of Clostridium tyrobutyricum.
Lysozyme is also added to beverages such as wine and beer to control Gram-positive spoilage bacteria like Pediococcus and Lactobacilli without interfering with fermentation. Beyond food preservation, its antimicrobial action is utilized in clinical settings, where the enzyme is an ingredient in pharmaceuticals such as throat lozenges and eye drops.
In a therapeutic context, lysozyme has been explored as a potentiating agent in antibiotic therapy, making bacteria more susceptible to conventional drugs. Furthermore, the enzyme is added to some infant-feeding formulas to resemble the protective composition of human milk. Modern research has also uncovered non-enzymatic applications; lysozyme crystals can generate an electrical charge when pressure is applied, a phenomenon called piezoelectricity. This property is being investigated for use in flexible electronics, such as power sources or coatings for implantable medical devices.